1. Field of the Invention
The present invention relates to a fixing unit used in an image forming device for thermally fixing a toner image onto a recording medium.
2. Description of the Prior Art
There has been known an image forming device, such as a copy machine or a laser printer, for forming an image on a recording medium using an electrophotography method. In this type of image forming device, a toner image is formed on, for example, a photosensitive drum and transferred onto a recording medium. Then, the toner image is fixed onto the recording medium by a fixing unit. More specifically, the fixing unit includes a heating unit and an electrical supply unit. The electric supply unit selectively supplies electricity to the heating unit to heat up and maintain the heating unit at a predetermined temperature. Heat generated by the heating unit is applied to the recording medium and fix the toner image onto the recording medium.
FIG. 1 is a time chart showing a conventional process for controlling the temperature of the heating unit. In this example, the heating unit is a heat roller with a halogen lamp housed therein. The halogen lamp is electrically connected to the electric supply unit via a trielectrode AC switch (TRIAC). The temperature of the heat roller is detected by a thermistor attached to a peripheral surface of the heat roller. The TRIAC is selectively turned ON and OFF to control the temperature of the heat roller based on the detected temperature.
Specifically, as shown in FIG. 1, when the TRIAC is turned ON, a lamp current is supplied to the halogen lamp and the halogen lamp starts generating heat. The TRIAC is maintained ON until the temperature of the heat roller reaches a predetermined temperature tb. When the temperature of the heat roller reaches the temperature tb, the TRIAC is turned OFF so that the halogen lamp stops generating heat. The TRIAC is maintained OFF until the temperature of the heat roller reaches to a predetermined temperature ta. Then, when the temperature of the heat roller drops to the temperature ta, the TRIAC is again turned ON. These processes are repeated to maintain the temperature of the heat roller between the temperatures ta and tb.
However, as shown in FIG. 1, immediately after the TRIAC is turned ON, a large current called a rush current flows into the halogen lamp. When a lighting fixture shares a power source with the image forming device, the rush current causes a voltage drop in the lighting fixture. When this voltage drop is generated at a frequency of about 8.8 Hz, for example, an unpleasant flicker is generated in the lighting. In recent years, some fixing units includes lamps as large as 10 kW. The larger the lamp, the greater the voltage drop in the lighting fixture, which results in even more sever flickering in the lighting. Therefore, controlling the generation of flicker accompanied with rush currents is becoming a major issue.